How to make a perfect Spanish omelette

There are few things in life that aren't improved by the addition of potato – a fry-up, a proper linguine alla genovese … even, if Delia is to be believed, a chocolate cupcake. Although it's not a vegetable with any great pretensions, the tuber's unassuming flavour is a passport into dishes in which fancy asparagus or exotic okra would stick out like a Jag-ful of WAGS at a village fete.

The Spanish omelette is a perfect example; comfort food par excellence, which transforms a couple of humble ingredients into a dish fit for el reino himself. On the face of it, this simple frittata-style recipe should present fewer issues than the classic French version: it's an altogether more rustic affair, which scorns the delicate jiggling and rolling practised across the border. But that doesn't mean that every Spanish granny dances to the same flamenco beat when it comes to her special tortilla de patatas.

The onion

Serious controversy even surrounds the inclusion of that most Spanish of ingredients, the onion. For some, introducing a pungent allium into such a delicately flavoured dish is a heresy tantamount to denying el Pulpo Paul's psychic powers. For others, well, it's utterly essential.

Using a recipe from Jenny Chandler's book The Real Taste of Spain, which declares vast quantities of olive oil "the key to an authentic" result ("weight watchers need not apply"), I make one omelette with a medium onion, diced, in addition to 700g waxy potatoes and 5 medium eggs, and one without.

Although the liberal use of oil (the vegetables are cooked in 300ml until tender, but not browned, and the omelette itself has a couple of tablespoons of extra virgin olive oil in it) saves the plain potato version from blandness, the onion, sweated down to melting softness beforehand, adds a sweetness which makes the other even better.

To caramelise?

Most recipes caution you not to let the onion colour – the only thing that should brown is the omelette itself, they say – but Jose Pizarro, the tapas mastermind behind Borough Market's Brindisa (and a member of a tortilla club who "meet to meet up regularly in different tapas bar or spanish restaurant to try out their tortillas"), caramelises them in his recipe, cooking them until "soft and brown" before adding the potatoes (floury, rather than waxy; also highly unorthodox).

The process takes longer, as the onions cook for 20 minutes by themselves, and then half an hour longer with the potatoes, but I like the extra sweetness the slow-cooked onion provides, although I'd prefer less of them, as the flavour is a little strident. The floury potatoes, however, are less to my taste – I like the solidity of the waxy varieties.

A good old soak

When making any sort of frittata, I generally add the beaten eggs to the other ingredients in the pan, so I'm surprised to discover almost every Spanish omelette recipe adds the cooked onion and potato to the jug of eggs and then pours them all in together. This makes more sense from a textural point of view, as it helps to stop all the potato sinking to the bottom, but real onion devotees advocate letting them sit for hours first, to allow the flavours to mingle. I try this, but find that the onion becomes overpowering after about 20 minutes – 10, while the omelette pan is heating, gives a more subtle result.

A crisp omelette

A couple of years ago, a friend returned from a trip to Barcelona claiming that, in their homeland, most Spanish omelettes were now made with crisps rather than freshly cooked potato. A native, she said, had informed her of the fact. I put this down to an overdose of rioja – until, in the course of researching this article, I came across Ferran Adrià's recipe. Adrià, is, of course, the Catalan regularly voted the world's best chef, whose wildly inventive cooking kick-started the entire 'molecular gastronomy' movement. He's also, it seems, at least partly responsible for this crime against crisps.

As some recipes call for the potatoes to be sliced as thinly as possible, ideally using a mandolin, and then deep fried, albeit at a fairly low temperature, the idea isn't quite as outlandish as it first appears. Take 60g good quality crisps (Spanish chef Jose Andres, a disciple of Adrià, calls for some cooked in olive oil, but despite the ever increasing girth of the posh crisp category, sunflower oil seems to rule the roost in this country, so I have to be content with some hand-cooked numbers sprinkled with natural sea salt), break roughly into pieces, and then add 2 beaten eggs and allow to sit for five minutes.

Pour in another couple of beaten eggs, season, and cook as normal. Either Adrià's crisps make all the difference, or the master's take on a national classic is not to my British taste, but I'm unimpressed: the crisps are so thin that they're all but invisible in this dull, slightly dry slab of solid egg – I like the potatoes to have a yielding softness, which this method could never hope to achieve.

A bit of a grilling

Personally, if I'm eating them straight away, rather than keeping them for a picnic or a sandwich, I like my tortillas to have that just set, baveuse quality so prized in a French omelette. But flipping a runny tortilla is hard work – invert it on to a plate, and you'll be left with a goodly quantity of liquid to tip back into the pan, which, unless you're exceptionally skilful, can spoil the shape of the finished dish.

Adrià, I notice, suggests sticking the omelette under a hot grill for a couple of minutes after it has browned underneath. This isn't a bad idea, especially if you're possessed of shaky hands, but I do think it gives a drier, more caramelised result, which I don't particularly like. If you make tortillas regularly, keep your eyes peeled for a special tool for the purpose, called a gira tortilla – they're sold in Spain, so it's not really cheating.

The soufflé effect

Fluffiness seems to be a quality much prized amongst omelette aficionados of all stripes, and, during a discussion online as to the best way to achieve this in the Spanish variety, a poster confides a Spanish chef of his acquaintance whisks the eggs whites separately in the manner of a soufflé omelette. I'm always suspicious of adding further steps to such an essentially simple recipe, but I give it a try, beating the eggs to soft peaks before folding in the beaten yolks, and then the cooked potato and onion. The cooked omelette certainly towers, but I find it too light – it lacks the richness which marries so well with the earthy flavours of potato and onion.

Healthy eating

I also test the theory that oil maketh the omelette, by sautéing my onions and potatoes in a couple of tablespoonfuls, rather than the 300ml suggested by Jenny Chandler. The end result isn't bad, but the potatoes in particular lack the silky richness which makes her recipe so delicious – and, after all, the vegetables are thoroughly drained afterwards, so it's not as if much of that oil makes it into the finished dish.

Whether you're an onion fan or not, a good Spanish omelette hinges on the quality of the potatoes – cook them low and slow in lots of olive oil, season them well and they'll carry the entire dish. You can play around with this recipe, adding shredded Serrano ham, or roasted piquillo peppers, or smoked paprika as you fancy – just don't try serving it to a Spaniard as an authentic tortilla espanola.

1. Heat the olive oil in a large frying pan over a medium flame, add the onion and cook gently for 20 minutes until soft and brown. Rinse the potato slices under cold water and pat dry. Add the potato to the pan – if it seems overcrowded, you can cook them in a couple of batches. Cook until the vegetables are tender and on the point of falling apart, then drain well, keeping the oil for your next omelette.

2. Add the potato and onion to the beaten eggs, season well, and leave to stand for 10 minutes, or longer if you prefer a stronger onion flavour.

3. Put a smaller pan (about 22cm) over a medium heat and add the extra virgin olive oil. Turn to coat, and then, when hot, add the mixture – it should almost fill the pan. Cook until it comes away from the edge of the pan, and looks about two thirds set.

4. Place a plate, or a saucepan lid, over the pan, and invert it so the tortilla flips on to the plate. Slide it back in, tipping any liquid egg in with it. Cook until it is springy to the touch: be careful not to overcook it: it should still be moist in the middle, even if you prefer it cooked right through.

Are you an onion denier or an allium aficionado when it comes to your Spanish omelettes? Do you serve them warm or cold and what are your favourite extra ingredients - can chorizo be beaten?